Alcator C-Mod Quarterly Report - 1QFY97

Following a short run campaign at the end of FY96, Alcator C-Mod started a maintenance period in the first quarter of FY97. The main activities during this three month period have been: continuing analysis of data from the winter and summer run campaigns; a complete inspection of the tokamak; upgrades to engineering and diagnostic hardware; planning and preparation for the spring 1997 campaign. The Alcator team was well represented at the IAEA and APS-DPP meetings which were held in Montreal and Denver, respectively. Scientific presentations from the Alcator group and collaborators numbered nine at the Montreal meeting, while in Denver there were a total of four invited talks and 37 contributed presentations.

This report is also available in pdf format.

Scientific Results

Results have been obtained across all the areas of investigation on the machine.

DIVERTOR/EDGE PHYSICS:

Three papers were presented at the IAEA conference in Montreal on divertor topics. A poster presented by B. LaBombard was titled `Transport Studies in the Scrape-off Layer of Alcator C-Mod'. It detailed the results of a transport analysis of the C-Mod scrapeoff layer. A key finding is that was independent of B and scaled as . This analysis primarily dealt with L-mode plasmas. H-mode plasmas had much steeper profiles, indicating lower 's. The second poster, titled `Impurity Screening Studies in the Alcator C-Mod Tokamak' was presented by G. McCracken on the topic of a series of impurity screening experiments. This was also covered by an invited APS paper, described in the next paragraph. The third paper was an oral contributed paper titled `Exploration and Control of Divertor Detachment in Alcator C-Mod', presented by B. Lipschultz, on the topic of the effect of divertor geometry on divertor detachment. It was shown that the vertical plate divertor utilized in Alcator C-Mod (and the basis for the ITER divertor) leads to a lower density onset for detachment than more open, flat-plate divertors (such as those presently on DIII-D and JT-60U). Through a simple model it was shown that the most likely reason for this difference is that the radiation profile is affected by the geometry. In particular, the vertical plate geometry allows for more radiation in the divertor.

A number of papers were also presented at the APS meeting in Colorado. Principal among the divertor related papers, was the invited talk by G. McCracken entitled `Impurity Screening in Ohmic and H-mode Plasmas in the Alcator C-Mod Tokamak'. Non-recycling impurities were injected at different poloidal locations. It was found that impurities were less likely to penetrate across the separatrix if injected at the inner wall or the divertor, as compared to the outer mid-plane. In addition, detachment increases the penetration of impurities into the core for all puff locations. Initial modelling of the results has been obtained with the DIVIMP code. The penetration appears to be a tradeoff between radial ionization location (diffusion time to the separatrix) and parallel flight time to the divertor plates.

Experimental results and modeling of thermoelectric currents in the SOL were presented at the APS meeting. Detailed modeling shows that over a well defined range of collisionality, an asymmetric divertor condition is necessary to satisfy the combined sheath fluid boundary conditions on heat flux and current flow. The collisionality range corresponds to that observed in the Alcator C-Mod when asymmetries are present.

Results from prototype tests of a novel divertor heat flux probe diagnostic in C-Mod were presented at the APS meeting (graduate student, Sanjay Gangadhara). The heat flux sensors appear to work reasonably well, providing a direct measurement (10 msec time resolution) of heat fluxes parallel to the magnetic field lines that intercept the outer divertor surface. The minimum detectable parallel heat flux is currently on the order of 20 MW/m . An array of 10 probes on the outer divertor surface will be used during the spring 1997 run campaign.

Work with the Alcator C-Mod edge database continued during this period as well. In particular, B. Lipschultz volunteered at the Montreal Divertor experts' meeting to organize an international effort to develop a common method for analysis and reporting of scrapeoff layer widths. A method was developed. Subsequently a draft memo was written detailing this method and other variables to be included in a test database before modifying the ITER database. This work is ongoing.

B. LaBombard has been working closely with the JET edge group on database issues. An exchange of data on SOL conditions (n, T , fitted ) between C-Mod and JET was performed (Kevin Erents, Guy Matthews - JET contacts). Similar to the results from C-Mod, JET sees an increase in across the SOL profile. They attribute the variation to the falling T profile. Both machines see no dependence of on B. The JET group is tentatively planning on showing combined JET and C-Mod SOL data at the upcoming EPS meeting. As a consequence of these data, a JET theorist (A. Chankin) is looking into the role of the collisional skin depth as a key parameter in regulating SOL transport. The results of the analysis are still under discussion.

As part of our collaborations with the U. Toronto group, developers of the impurity transport code DIVIMP, Peter Stangeby and his student, Steve Lisgo visited MIT after APS. A series of discussions were held regarding future impurity transport experiments at MIT and coordination with modelling at U.T. An effort has been made at MIT to increase the diagnostic information useful to DIVIMP. The code has been ported to run locally on the DEC Alphas, specifically for modelling impurity transport in the C-Mod boundary plasma. It is intended that the code be used in close connection with real experimental measurements of measured edge conditions as inferred from probes, spectroscopy, gas gauges and the residual gas analyzer. This is the full-time research topic of a graduate student, Vitali Belevtsev. Preliminary results for testing purposes with C-Mod geometry have been attained, using carbon as the primary impurity. Now that the code is operational, more realistic plasma conditions and impurities will be used. Particular attention will be paid to extrinsic impurities, such as nitrogen, neon and argon, since these are of primary interest for radiative divertor experiments.

Progress is continuing on the development of a 2-D ``interpretive" (TRANSP-like) edge transport code (graduate student, Maxim Umansky). 2-D T solutions using a reduced set of Braginskii transport equations can now be readily computed for any discharge in which EFIT output is available. values are found that best fit the experimental data. Comparisons between the 2-D code and standard 1-1/2 D ``onion skin" analysis (J. Kesner, Phys Plasmas, 2, (1995) 1982, B. LaBombard, et al., Phys Plasmas 2, (1995) 2242) are underway.

Experiments have been done on Alcator C-Mod with impurity gas puffing into ICRF-heated H-mode plasmas to investigate the dissipative divertor concept envisaged for a high-power fusion reactor. In C-Mod high q-parallel H-mode plasmas, divertor detachment has been achieved with nitrogen puffing, while neon and argon puffing did not result in detachment. This is in contrast to the L-mode situation, where detachment was achieved with all impurity gases as well as with deuterium puffing alone. In the H-mode cases, nitrogen puffing reduces and increases the radiation in the divertor, which are both necessary for detachment with high q-parallel. Plasma performance in H-mode is affected by impurity puffing and detachment. The H-factor for energy confinement, relative to the ITER-89P scaling, remains above 1.5 with nitrogen puffing; it is degraded even more with neon and argon puffing. The change in Z was in the range from 0.5 to 1.0 during impurity puffing. The peak in the divertor volumetric emissivity remains outside the separatrix after detachment. This implies an efficient use of divertor volume for power dissipation, in contrast to the situation in L-mode, where the radiation moves up to the x-point and in some cases inside the separatrix.

Significant volume recombination of the majority ion species in the C-Mod divertor region has been observed. We believe that this is the first direct experimental observation of this process in a tokamak with high input power. Analysis of the visible spectrum around the Balmer series limit shows that T in the recombining plasma is 0.8-1.5 eV and that n is about  m . Further research on the scaling and spatial distribution of recombination is planned for the next campaign. A paper on this subject has been submitted for publication in Physics of Plasmas.

CCD images from the C-Mod plasma TV system, filtered for D emission, have been inverted to yield high resolution, 2-D pictures of this emission. The inversion algorithm, which processes about 20,000 chords of brightness data, takes only about 5 seconds of CPU time per image. We plan to utilize this work with other filtered TV images.

Two papers were presented at APS from our collaboration with the Univ. of Maryland (B. Welch, H. Griem), involving high resolution visible and UV spectroscopy. They were titled: ``High-Resolution Visible/Ultraviolet Measurements from the Alcator C-Mod Tokamak" and ``High Resolution Measurements of Zeeman Patterns in Visible/Near Ultraviolet Spectra from the Alcator C-Mod Tokamak". These posters described initial velocity and temperature measurements of neutral deuterium and calculations of the expected Zeeman patterns along the line-of-sight of the optical system. The results show narrow D line widths, which, if thermal, would correspond to neutral temperatures of around 1 eV. Line shifts have also been observed, typically during the current ramp-up.

We have continued to analyze dust removed from the first wall tile surfaces as part of an ITER support activity. Scanning electron micrographs, together with energy dispersive spectroscopy (EDS) was used to determine the composition of the dust. Though most of the material found was molybdenum dust, easily detectable amounts of Cu, Si, Fe, Al, and Ca were also found. A histogram of particle sizes was generated indicating an average particle size of 72 . This analysis is continuing.

CORE TRANSPORT

We have continued our studies of the role of edge parameters in H-mode physics. This work was reported in detail at the IAEA meeting. Both controlled density scans and a database of edge parameters for many C-Mod H-modes have been analysed. It is found that a minimum edge temperature, as measured by electron cyclotron emission diagnostics, must be exceeded in order to enter and remain in H-mode. This condition is not always sufficient, however. Density, and therefore pressure, appear to play a less important role. Comparison of 5.3 T and 8 T discharges shows that the threshold temperature increases with B . The ion larmor radius thus remains in a narrow range at the transition, while beta and collisionality show a very wide variation. Discharges with the ion grad-B drift away from the x-point have L-mode temperatures similar to those with the usual direction toward the divertor. Both the global power threshold and local edge temperature at the L-H transition are approximately doubled. We noted with interest that qualitatively similar results regarding the importance of edge parameters in H-mode were reported at the IAEA meeting by both ASDEX-Upgrade and DIII-D. A collaboration has been started between C-Mod and ASDEX-Upgrade personnel to compare data more quantitatively and to carry out dimensionlessly similar experiments. Amanda Hubbard will be visiting Garching Feb 10-14 as part of this work. Already, examination of ELMy discharges from the summer experimental campaign has shown that Type III ELMs only occur at relatively low edge temperatures. This confirms similar observations by Zohm on ASDEX Upgrade. A study of pedestal parameters in ELM-free and Enhanced D discharges is ongoing. In response to such results, ITER is now planning to start a new inter-machine ``Edge Operational Space Database" with a view to predicting more accurately the H-mode threshold and performance for ITER. C-Mod personnel have supplied preliminary data and are in frequent communication with Drs. Post and Janeschitz of the ITER JCT regarding plans for setting up this database, choice of parameters etc. We intend to submit experimental data as soon as the database is ready.

A critical issue for ITER is the width of the H-mode pedestal. The width on C-Mod is narrower than on other machines (< 1 cm) and our measurements are currently resolution-limited. Significant diagnostic improvements such as high-resolution x-ray and bolometer arrays and a heterodyne radiometer are being made to improve the precision of these measurements. Due to the compact size of C-Mod, our data should have considerable leverage in determining the size scaling of the pedestal width. We hope to have new results from the upcoming experimental campaign.

Work on the H-mode threshold database and extrapolation to ITER was presented by J. Snipes at an Expert Group meeting in Montreal. The results confirm the dimensionless scaling analysis of the ITER H-mode threshold with global parameters through regression analyses across all tokamaks in the ITER H-mode threshold database. Initial results were also presented taking into account the total radiated power and the available edge density and temperature measurements in the database.

In initial coordinated experiments to study dimensionless scaling, C-Mod and DIII-D discharges with nearly identical shapes and scaled global parameters have been compared. Good agreement is found for the scalings of many profile parameters, including T , T , n , and effective thermal diffusivity. These experiments will be expanded to include more detailed scalings with DIII-D as well as comparisons between C-Mod and JET, ASDEX-U and JT60-U.

Large central toroidal (impurity) rotation velocities have been observed in ICRF H-Mode plasmas. Rotation velocities up to  cm/sec have been observed, in spite of the fact that these plasmas have no direct momentum input. The rotation velocities are seen to correlate with stored energy increases.

Analysis continues on quantifying the types of ELMs observed on C-Mod together with some mode analysis of their magnetic precursors. For those H-modes which exhibit ELMs, Type III are the most typical, with a frequency that decreases with increasing input power (range from 1 to 6 kHz). Type III ELMs often have oscillating magnetic precursors with frequencies in the range of 100 - 160 kHz and high m and n numbers, both typically around 9 or 10. After boronization, Type III ELMs became more rare and another kind of H-mode, an Enhanced D alpha H-mode was found. In this mode, the overall D alpha emission increases again after the initial drop into H-mode. The emission can reach L-mode levels even though the plasma remains in H-mode. The enhanced D alpha emission shows no magnetic ELM activity over the range of frequencies observed (up to 500 kHz). In some cases, during Enhanced D alpha H-modes, there are events on the D alpha and edge ECE signals that may be compared to Type II ELMs, since they are rather chaotic in time with small amplitude, reminiscent of ``Grassy" ELMs. At the APS meeting, J. Snipes presented a talk on ``Enhanced D alpha H-modes in Alcator C-Mod" suggesting that this may be a promising operating regime for ITER because it combines steady state high confinement with low core radiation and low impurity accumulation. Recent analysis of fast magnetic signals (I. Hutchinson) has shown the presence of a high frequency bursting edge mode (  kHz, n=1, ) which may be playing an important role in the dynamics of these discharges.

Theoretical and numerical analysis of the Enhanced D H-modes and associated MHD activity are being undertaken by Freidberg, Kesner, and Ramos. They have analysed the MHD stability of several C-Mod discharges using a procedure developed by Wolfe [Bull APS, Vol 41, Paper 5F9, (1996) 1482.] which utilizes kinetic profiles in EFIT and does a stability analysis using DCON [supplied by A. Glasser]. They are in the process of analysing the stability of these discharges using the KINX and CAXE ideal MHD codes [S. Medvedev, L. Villard, L.M. Degtyarev, A Martynov, R. Gruber and F. Troyon in Proc. 20th EPS Conf. on Cont. Fus. and Pl Phys, Lisbon, 1993. pt IV, p. 1279] as well as the MARS resistive code [supplied by Bondeson].

The morphology and the physics of the sawtooth oscillations observed on Alcator C-Mod have been analyzed and a poster presented on this topic at the APS meeting by F. Bombarda. Sawteeth are almost always present. They can vary from small, rapid oscillations, to the large sawteeth observed during RF H-modes, where the interval between crashes at the center can exceed 25 msec, several times the ohmic value, and the drop of central electron temperature at the crash can be 2 keV. The period is observed to be nearly independent of density, increasing with plasma current and ICRF power. A good correlation is found between the sawtooth period and the plasma stored energy over the range from 20 to 200 kJ. Theoretical studies of sawteeth in C-Mod have been carried out by Bonoli, and Kesner. A sophisticated model has recently been developed by Porcelli and coworkers [F. Porcelli, B. Boucher and M. Rosenbluth, to be published in Plasma Physics and Cont Fusion, 1996] with the goal of utilizing this model in the design of ITER. The Porcelli model predicts the threshold and relaxed profiles of a sawtoothing discharge. In collaboration with Porcelli, Bateman and Kinsey, this model is being installed into the BALDUR code. Simulations of chosen C-Mod discharges will be utilized to ascertain the accuracy of this model in predicting sawtooth behavior in C-Mod. Porcelli has visited MIT for detailed discussions on this collaboration.

A multi-stage amplifier with improved time response was added to the He neutron detector system. This allowed better resolution of neutron sawtooth activity while avoiding saturation of signals at high neutron rates which had occurred in earlier sessions. These data show clear sawteeth on the neutron rate, with crashes falling to between 10% to 40% of the peak value. Compund sawteeth and MHD activity evident on electron temperature data (ECE) are reflected by flattening of the rate of the rise in the neutron rate. The ``crash" of the neutron rate at the electron temperature crash is significantly slower than the electron temperature fall, with a time constant between 0.3 ms and 0.6 ms. These data are being examined in detail to look for correlations with other plasma parameters.

In an atomic physics study of the spectrum of He- and Li-like nitrogen, a line resulting from inner shell excitation of Li-like nitrogen has been observed. Its intensity, relative to the neighboring He-like N lines, has been shown to depend sensitively on the transport change occurring at the H- to L-mode transition. The experimental determination of the wavelength of this line has re-opened a question which was thought to be have been answered over 25 years ago.

Laser ablation injection of trace impurities has been extensively used to determine impurity transport coefficient profiles. Impurity diffusion and convection velocities are found to be anomalous in L-Mode plasmas, while in H-mode (particularly ELM-free), impurity confinement times are long, and the transport coefficients approach the neoclassical values at the plasma edge.

In a collaborative effort (with R. Neu), tungsten x-ray spectra from ASDEX-U have been analyzed. X-ray lines between 7 and 10 Åhave been identified as being from charge states between Kr-like and Ni-like, through comparison with calculations performed by Kevin Fournier at LLNL.

Recent analysis of neutral dynamics in C-Mod discharges points to a non-negligible role of the neutrals in the edge physics, especially with regard to H modes. This could help to explain some important aspects of that regime, in particular with regard to power balance. It could also help to elucidate some of the differences observed amongst the different machines and should contribute in the extrapolation towards ITER.

RF PHYSICS

Reliable high power ICRF heating at the 2.5-3MW power level (maximum of 3.5MW) has enabled studies of improved confinement modes and high heat flux divertor experiments with . Boronization of the molybdenum wall has significantly reduced the radiated power from the main plasma, resulting in better H-modes and successful heating in low single-pass absorption schemes including He minority heating at 8T. In particular, quasi-steady-state H-modes with H=2 and have been obtained simultaneously with and , which became possible because of the shorter particle confinement time. In PEP modes with high central density, ion heating becomes the dominant heating channel. Highly localized ( ) electron heating profiles were produced by both on-axis and off-axis mode conversion heating. Efficient electron heating during the initial current ramp-up phase was demonstrated; this will be necessary for studies of reversed shear plasmas. Recent developments in full-wave codes have improved the agreement between the observed experimental results and the theoretically calculated power absorption profiles (toroidal full-wave code TORIC) and power partition between ions and electrons (bounce-averaged Fokker-Planck code FPPRF). Feasibility studies of fast wave current drive were carried out using a sophisticated MHD equilibrium and current drive code (ACCOME), which showed that with 3MW of 60MHz rf power, up to 0.5MA of current can be sustained at 5.3T with central fast wave current drive and off-axis bootstrap current ( ). Detailed modelling studies indicate the possibility of sustaining C-Mod plasmas in steady state (for time scales longer than the skin time and the L/R time) near the beta limit ( ) at , , and with the addition of a 4.6GHz LHCD system.

Engineering

A major effort during the maintenance period has been the disassembly, inspection, and reassembly of the machine. Before we could begin reassembly several engineering changes had to be made to address issues brought to light, either during the inspection, or earlier during the run period. All the OH coaxes were modified so that the full compliance of the springs in the coax foot could be realized. Verifying that the proper changes were made involved electro-mechanical simulation runs combined with extensive mechanical tests of the coax itself.

The inspection indicated that the upper TF arms were moving inward toward the TF core farther than we had expected. A new shimming arrangement has been developed to limit this movement. The inspection also indicated wear on the anti-torque pins used to restrain OH stack movement relative to the vacuum vessel. These pins were enlarged in size to distribute the forces over a larger surface area.

Many upgrades to engineering systems were effected during the last quarter. New reactors to protect the EF4 supplies have been modelled, specified, and ordered. Improvements have been made to the fault detection circuits for most of the magnet supplies. New CAMAC has been added so that better analysis of power supply operation can be made, and problems debugged faster and more reliably. New overvoltage boards using breakover diodes are being developed and should be installed for the next run period. PLC boards are being installed to monitor instrumentation power supplies voltages on all critical control systems.

Much has been accomplished on our RF systems. The new tuneable transmitter installation is well along. We have installed and made modifications to the driver and final units. We have installed power and water to the new systems, and nearly completed installation of the control and fault detection systems. A great deal of work has been expended so far in construction of new crowbar circuits for the new transmitters. This work is nearing completion. Coax runs between the new transmitters and the experimental cell are well underway. Installation of the coax in the cell has begun. PLC software and hardware for controlling the new equipment is nearly complete. We have also made improvements to the primary 13.8kV power distribution system that should provide more reliable operation for all the RF systems.

Work on the new diagnostic neutral beam, a major component of our collaboration with the University of Texas (FRC), is moving along quickly. The beamline, support electronics, and power supplies were moved from the University of Texas. We have installed the supplies in our power room and completed their power, ground, and water distribution systems. We are currently focusing work on design changes to the high voltage systems including the primary high voltage supply, and the modulation/regulation circuitry. A mechanical support for the beam, which will be used at least through initial reassembly and testing, is under construction at the University of Texas. Completion of the support will be immediately followed by reassembly of the beamline and vacuum testing. Preliminary designs for the DNB diagnostics (BES and CHERS) have been completed.

Upgrades

Our Johns Hopkins University collaborators (Finkenthal, Regan, May) are installing a compact, multilayer-mirror-based, XUV monochromator, which will be located in the C-Mod vacuum vessel and will observe resonance line emission from nitrogen. This will provide important new information for use during our dissipative divertor studies.

Our collaborators from LANL (G. Wurden, R. Maqueda) have designed an IR periscope which will view the C-Mod divertor region from above. They are also supplying an IR camera for use in studying heat deposition on the divertor plates and in studying heat deposition after or during disruptions.

An additional piezoelectric valve was installed in a vertical port near the floor of the divertor. This valve will be used for fast control of impurity puffing into the private flux region of the divertor.

An RGA was installed with access to the private flux region of the divertor. This RGA has a fast (  msec) sampling rate for up to four different (m/e)'s, simultaneously. This RGA will be used to inventory the impurity atoms puffed into the vacuum vessel, as well as to monitor intrinsic low-Z impurity species.

Various components of the core Thomson scattering diagnostic system have been upgraded and the overall reliability of the system has been improved. The laser optical layout has been modified to achieve better beam quality, a new laser beam tracking system has been built and remote control of the beamline has been implemented, allowing the alignment of the beam to be controlled and adjusted from the control room. The arrangement of detection channels has been modified to provide for better coverage of the plasma temperature and density profiles.

Improvements have been made on the omegatron Z/M ion spectrum probe, including a rebuilt RF cavity, an improved probe bracket, and improved in-vacuum cabling. The omegatron will be installed shortly and should be fully operational during the spring 1997 run campaign.

Progress has been made on modifying the F-bottom fast-scanning probe drive to inject trace amounts of impurity gas deep into the scrape-off layer (up to the separatrix). In addition, progress has been made on the design and fabrication of two optical imaging diagnostics to record light from impurity line-emission ``plumes". These systems will be used to deduce local parallel and cross-field transport of the injected impurities. Combined with information from core spectroscopy diagnostics and the Omegatron impurity Z/M spectrometer, the system may also be used to assess directly the screening of the SOL for extrinsic, non-recycling impurities.

As part of the MIT-Texas collaboration, Roger Bengtson (U. Texas) has made progress assembling components for a horizontal fast scanning probe, optimized for fluctuation studies. The probe will be installed for the spring 1997 run campaign. Plans are also well along for the addition of an enhanced ECE diagnostic system to measureme core T fluctuations, a joint collaboration with Auburn U. and U. Texas.

In collaboration with PPPL an extensive upgrade of our reflectometer is underway. Components are being added to stabilize the Gunn diodes, improve fluctuation measurements, allow the system to be switchable between O-mode and X-mode operation, and at least one higher frequency channel is being added, probably at 140 GHz.

A tangentially viewing interferometer is being developed for installation for the next run campaign. This new diagnostic will use a diode pumped, frequency doubled, CW, YAG laser to provide the two color beams needed for vibration subtraction. The initial system will provide only one chord, but we plan to upgrade to more channels if the system works well. In addition, PPPL in collaboration with PSI, is developing a very high spatial resolution frequency doubled interferometer for the plasma edge.

One of the x-ray arrays has been modified to view the edge plasma region with very high spatial resolution (1 mm). The idea is the determine how the H-mode pedestal width scales with , and/or ELM behavior (e.g. ELM-free, enhanced D-alpha, etc.)

A fiber-optic feedthrough with 26 fiber feedthroughs has been designed by S. Pitcher and B. Lipschultz. 14 of these feedthrough fibers will be used for a toroidally viewing array for He line ratio measurement of and as well as doppler shifts ( ) and widths ( ). Another 6 fibers will be utilized for a toroidal view of the FSP gas injection for similar measurements. The remainder of the fibers will be used for views of the different antenna components which have, up until now, been out of the field of view of the diagnostic ports. This latter diagnostic will be used for Mo source rate measurements during H- and L mode experiments.

We are presently installing the hardware for a helium ``beam" boundary diagnostic, for measuring plasma density and temperature at the outside midplane. Helium gas is released from a capillary tube in the middle of the AB limiter, which is usually within several centimetres of the last closed flux surface (LCFS). The spectral emission from three HeI lines in the near-IR is tangentially observed using an in-vessel telescope mounted on the wall with 14 spatial channels. The radial resolution of the system is expected to be  mm. The three spectral lines, at 14 radial locations, will be measured with the Chromex spectrometer, which gives a time resolution of 50 ms. From the ratio of the spectral intensities, the local plasma density and temperature can be derived. This system is similar to one pioneered recently on TEXTOR. Should preliminary tests in the next run campaign prove successful, then a dedicated fast spectrometer will be used with 1 ms or better time resolution. This beam will also be used for T and v measurements in conjunction with the U. Md. group.

In order to investigate the role of neutrals we are adding a variety of diagnostics to help quantify their effects. We are fielding a low energy neutral particle energy analyzer, based on the time-of-flight technique, which should give us the neutral energy distribution from  20 eV to  4 keV. In addition, we are planning to add several XUV diode arrays, which should give us profiles of radiated power. These new detectors, combined with the information from conventional foil bolometers, should enable us to quantify the power carried by neutral particles as distinct from photon radiation.

Conceptual design of a neutron profile monitor (also known as a neutron collimator) is continuing. This diagnostic aims at measuring the profile of the D-D fusion rate, and has the potential to yield important information on the ion temperature profile and consequently on the core transport. Immediate issues relate to the compactness of Alcator C-Mod tokamak which influences the neutron scattering and signal levels. We plan to field a variety of detectors to evaluate their effectiveness in preparation for a full design.

The quartz crystal in one out of the five high resolution X-ray spectrographs (HIREX) has been replaced with ADP, in order to allow for spectral measurements of the most important tungsten x-ray lines in the vicinity of 6 Å.